1. Field of the Invention
The present invention relates to a fuel cell for controlling a fluid necessary for power generation, and a fuel cell system having a plurality of fuel cells.
2. Description of the Related Art
Recently, a fuel cell which generates power by reacting a fuel such as hydrogen with an oxidizer such as oxygen has been utilized as a clean power source having a high energy conversion efficiency.
Such a fuel cell has conventionally had a structure in which a membrane electrode assembly (MEA) for reacting a fuel with an oxidizer is sandwiched between separators for separating the fuel from the oxidizer and individually supplying these to the membrane electrode assembly. There is also known a fuel cell in which a plurality of cell stacks in each of which a plurality of fuel cells each having this sandwiched structure are stacked are connected to each other, as disclosed in Unexamined Japanese Patent Application KOKAI Publication No. 2000-243421.
A fuel cell in which a plurality of such cell stacks are connected is provided with valves for adjusting the amount of fuel and the amount of gas as the oxidizer both respectively supplied to each cell stack, in order to improve the power generation efficiency of the whole fuel cell. The fuel cell controls the opening or closing degree of each valve based on the output voltage of each cell stack and the air pressure applied to each cell stack, thereby adjusting unevenness of the power generated by each cell stack.
However, such a fuel cell apparatus has a problem that the fuel cells in each cell stack cannot be supplied with air of a well-balanced amount, because only one valve provided externally per cell stack collectively supplies air to the plurality of fuel cells in the cell stack. Furthermore, a voltage sensor senses the voltage per cell stack but cannot determine whether unevenness is caused in the voltages of the respective fuel cells in the cell stack, and this makes it harder to control the plurality of fuel cells in the cell stack with a fine balance.
Unless the fuel and oxidizer are uniformly supplied to the respective fuel cells, there arises a problem that the fuel cells will have different power generation performances, and the power generation efficiency of the whole cell stack degenerates as influenced by a smaller power.
In addition, the fuel cell easily loses an equilibrium suitable for the fuel and the oxidizer to react with each other, when the supply amounts of the fuel and oxidizer to be supplied to each cell only minutely increase or decrease. Therefore, there occurs a problem that the power generation efficiency greatly drops, if the supply amounts of the fuel and oxidizer to be supplied to each cell are not adjusted.